Field of the Invention
The present invention relates to wireless communication, and more particularly, to a D2D signal transmitting method implemented by a terminal in a wireless communication system and a terminal using the said method.
Related Art
In International Telecommunication Union Radio communication sector (ITU-R), a standardization task for International Mobile Telecommunication (IMT)-Advanced, that is, the next-generation mobile communication system since the third generation, is in progress. IMT-Advanced sets its goal to support Internet Protocol (IP)-based multimedia services at a data transfer rate of 1 Gbps in the stop and slow-speed moving state and of 100 Mbps in the fast-speed moving state.
For example, 3rd Generation Partnership Project (3GPP) is a system standard to satisfy the requirements of IMT-Advanced and is preparing for LTE-Advanced improved from Long Term Evolution (LTE) based on Orthogonal Frequency Division Multiple Access (OFDMA)/Single Carrier-Frequency Division Multiple Access (SC-FDMA) transmission schemes. LTE-Advanced is one of strong candidates for IMT-Advanced.
There is a growing interest in a Device-to-Device (D2D) technology in which devices perform direct communication. In particular, D2D has been in the spotlight as a communication technology for a public safety network. A commercial communication network is rapidly changing to LTE, but the current public safety network is basically based on the 2G technology in terms of a collision problem with existing communication standards and a cost. Such a technology gap and a need for improved services are leading to efforts to improve the public safety network.
The public safety network has higher service requirements (reliability and security) than the commercial communication network. In particular, if coverage of cellular communication is not affected or available, the public safety network also requires direct communication between devices, that is, D2D operation.
D2D operation may have various advantages in that it is communication between devices in proximity. For example, D2D UE has a high transfer rate and a low delay and may perform data communication. Furthermore, in D2D operation, traffic concentrated on a base station can be distributed. If D2D UE plays the role of a relay, it may also play the role of extending coverage of a base station.
Meanwhile, a D2D operation can be classified into mode 1 and mode 2. Mode 1 is a mode in which a resource for the D2D operation (e.g., transmission of a D2D signal) is scheduled from a base station. In order for a terminal to transmit the D2D signal by mode 1, the terminal needs to be in an RRC-connected state.
Mode 2 is a mode in which the terminal directly selects the resource for the D2D operation (e.g., the transmission of the D2D signal). The terminal directly selects the resource for transmitting the D2D signal in a resource pool.
However, when the D2D signal transmission by mode 2 is allowed only for the terminal which is RRC-connected, there may be a problem in terms of continuity of the D2D operation.
For example, it is assumed that the terminal performs the D2D operation (in more detail, D2D communication) outside network coverage and enters the network coverage. The terminal is in an RRC-idle state immediately after entering the network coverage and when the D2D communication is not allowed in the RRC idle state, the D2D communication stops. Since stability and reliability are important in the D2D communication for public safety, occurrence of the stop may become an issue.